Multi-stage hydraulic pressure boosters for use in vehicular braking systems are relatively well known in the prior art. Such pressure boosters, or intensifiers, employ a system of concentric pistons to obtain a gradual boost in wheel cylinder pressure, providing a brake pedal feel comparable to that provided by vacuum-power assisted vehicular braking systems. Such prior art pressure intensifiers are illustrated in U.S. Pat. Nos. 3,101,282 and 3,010,238 to Jansson, and in U.S. Pat. Nos. 3,425,222; 4,976,190; and 5,048,397 to Cooney. These references disclose a self-contained apparatus installed along the brake fluid line extending between the master cylinder or within the master cylinder housing and the wheel calipers of the vehicle. A cylindrical chamber surrounds two concentric pistons slidably disposed in the cylinder and urged by a compression spring toward a fluid inlet of the cylinder. A spring-loaded valve is disposed inside the inner piston and is held open by engagement with an abutment within the cylinder.
Application of pressure to the brake pedal by a user displaces hydraulic fluid from the master cylinder into the intensifier which initially passes directly through the intensifier into the caliper urging the brake pads into engagement with the rotating disk or rotor, or; in the case of drum brakes, with the drum. Once the brake pads engage, the resistance to further movement increases the hydraulic pressure throughout the system and at the inlet of the intensifier sufficiently to cause the pistons to move against the spring force thereby closing the valve, preventing any further flow through of fluid through the intensifier. The ratio between the inlet and outlet side of the pistons of the intensifier are such as to deliver greater hydraulic pressure to the caliper than the pressure exerted on the intensifier by the master cylinder, hence providing the desired boosting or intensifying effect.